Eleusinia

"In Grecian antiquity, a festival kept in honour of
Ceres, every fourth year by some states, but by others every fifth. The Athenians celebrated it
at Eleusis, a town of Attica, whence the name.

It was celebrated with a world of ceremony, and persons of both sexes were initiated in it ;
it being deemed impious to neglect doing so. The mysteries were of two sorts,
the lesser and the greater ; whereof the former were facred to Proserpina, Ceres's daughter, and the latter to Ceres herself. According to Lactantius, they consisted in a mythical representation of what mythologists teach of Ceres ; though some of the Christian fathers will have the great mystery, or secret, which they were forbidden by law, upon pain of death, to divulge, to have been the representation or figures of both male and female privities, which were handed about and exposed to the company". -- Encyclopædia Britannica 1771.